The ability of zinc to inhibit corrosion of ferrous metals is well known. Accordingly, soluble zinc salts are vital ingredients of many corrosion treatment programs. An art recognized problem associated with zinc-containing treatments, particularly In cooling water, is the uncontrolled precipitation of zinc salts. For example, the use of orthophosphate in combination with zinc as a cooling water treatment is known. When orthophosphate and zinc are both present in an aqueous system, zinc phosphate precipitation becomes a concern. Precipitation of zinc in other forms, for example zinc hydroxide or zinc silicate can also occur. The solubility of the various salts, that is, the retention of the respective salt constituents in ionic form depends upon such factors as water temperature, pH and ion concentration. In alkaline waters, particularly above about pH 7.5 dissolved zinc tends to deposit out or drop out. Zinc salts are also known to be unstable in neutral or alkaline water and will precipitate with phosphates. Thus, if any of these conditions are present, the aqueous medium becomes prone to zinc precipitation. With the formation of zinc scale, many of the surfaces in contact with the aqueous medium will foul and the amount of effective (soluble) corrosion inhibitor present in the aqueous medium can be significantly reduced.
Attempts to control the stability problems commonly encountered in zinc-based water treatments have typically involved pH adjustment or stabilizers which tend to reduce corrosion inhibition efficacy. Most often alkalinity or acidity is employed to achieve even minimal stability for zinc-based treatments. In most cases of blends of zinc and polymers, precipitates (accelerated at low temperature, i.e., less than about 45.degree. F.) composed primarily of zinc and calcium complexes form. Many additives have been investigated such as chelants and threshold inhibitors to alleviate precipitation problems in such products. Attempts to stabilize a neutral pH formulation with these types of additives have resulted in failure or stabilized formulations which have significantly reduced corrosion inhibition efficacy. The loss of corrosion inhibition efficacy is believed to be primarily due to insoluble zinc complex formation prior to product use.
Zincates are generally observed to be soluble and stable at low pH values (less than about pH 5) and often under highly alkaline (pH greater than 12) conditions. When the zincates are used in combination with polymeric materials which are stable at higher pH's such as poly (ether carboxylates) which are stable at pH's greater than 5, the expected pH range for stability of the mixture is pH 4 to 5. A neutral aqueous mixture would not be expected to show stability. The stability problems related to treatment component matrix and final product pH have made field application of certain zincate corrosion/deposition inhibitors difficult. Scaling of feed lines and equipment is not uncommon. The use of alkalinity or acidity to attempt to enhance product stability has been of limited success.